1. Field of the Invention
This invention relates to a method of ethanol extraction of corn fiber that has been thermochemically treated.
2. Description of the Background Art
“Wet-milling” is a process by which corn can be converted into ethanol, corn sweeteners, and starches. As used herein, “corn fiber” is defined as “the product obtained from the wet-milling process, which involves an initial steeping of corn kernels in aqueous sulfur dioxide at an elevated temperature followed by gentle grinding and physical separation of the outer fiber layers from starch, protein and other components.”
Corn fiber is produced by corn wet-milling at the rate of 13% per bushel of corn processed. More than 15,000 tons of corn fiber are produced per day at wet-mills according to the Corn Refiners Association. Along with the corn fiber, portions of the protein, oil, and starch from corn are separated with the fiber stream, and, with corn steep liquor and stillage that are sprayed on the fiber along with corn germ meal, which is mixed into the stream, a total of about 25% of the corn processed becomes part of the corn gluten feed stream. Corn steep liquor is the liquid that is derived from the initial steeping of the corn kernels, and stillage is the bottoms from the distillation of the ethanol fermentation broth, comprising mainly water, non-volatiles, and solids derived primarily from yeast cell mass. The corn steep liquor and stillage provide nutrients and protein to the corn gluten feed, which is a low value by-product stream that is sold as animal feed. In the present invention, a cost effective method is provided that upgrades the corn fiber stream into more valuable and usable components.
Corn fiber is composed of approximately 15-25% starch, 10-13% protein, 33-42% hemicellulose, 15-18% cellulose, 3-6% ash, 3-6% oil, and 1-2% other components. The hemicellulose is composed of 50-55% xylose, 30-35% arabinose, 4-6% galactose, 3-5% D-glucuronic acid and 2-5% other components including mannose, coumaric acid, and ferulic acid. The corn fiber stream at the plant coming from the dewatering presses contains 30-50% solids. During thermochemical treatment of the corn fiber, the fiber is heated to 130-180° Centigrade (C.), which solubilizes the starch and hemicellulose fractions, while leaving the cellulose intact. Starch is composed of two types of glucose polymers, amylose and amylopectin. Amylose is a linear polymer with the glucose molecules linked by α-1,4-glycosidic bonds, and amylopectin is a highly branched polymer with the glucose molecules linked by α-1,4-glycosidic bonds with α-1,6 linked branches. Hemicellulose in corn fiber is composed of a β-1,4 linked xylose backbone with side-chains composed of arabinose, xylose, galactose, glucuronic acid, mannose, ferulic acid, and coumaric acid. The starch can be removed from the fiber by hydrolysis with a combination of heat and either starch-degrading enzymes or sulfuric acid. Under these conditions, the starch polymer is hydrolyzed first to soluble oligosaccharides, which can be further hydrolyzed to glucose by a secondary acid or enzyme hydrolysis step. The hemicellulose can be partially hydrolyzed by treating the corn fiber at temperatures above 121° C., but the complete hydrolysis of the xylan backbone to monomers requires further treatment with acid or enzymes.
The hydrolysis of the starch and the hemicellulose can also be combined into a single step. The native corn fiber, containing residual sulfur dioxide from the steeping process, can be treated at high temperatures with the optional addition of acid. This treatment will cause simultaneous hydrolysis of the starch and the hemicellulose.
The monosaccharides from the hydrolyzed starch and hemicellulose can be used in many different industrial applications including fermentations and catalytic conversion to sugar alcohols and subsequently polyols. The glucose from the starch can be used in a yeast fermentation to produce ethanol, or can be fermented to other products. The xylose can also be similarly fermented to a number of fermentation-derived products known by those persons skilled in the art. The ferulic acid can be used as a feedstock for the production of vanillin.
The remaining corn fiber after the hydrolysis step can then be contacted with a solvent to extract the oil present. For example, U.S. Pat. No. 5,843,499 (Moreau at al.) discloses that the oil fraction, which contains phytosterols, can be extracted from corn fiber using hexane in the presence of the antioxidant, BHT. The process described in Moreau et al. was completed on dried, ground corn fiber at room temperature with agitation. The extraction set forth in Moreau et al. resulted in mixed oils containing triglycerides (TAG), fatty acid esters of phytosterols (St-FA), free fatty acids (FFA), tocopherols, free phytosterols (St), and ferulic acid esters of phytosterols (St-F). Moreau et al. reports that the total percentage of oil in normal corn fiber to be as high as 3.33 wt %. Further, Moreau et al. reported approximately 18% (wt/wt) total sterol content (St-FA, St, and St-F) in the extracted oil with 8% as St-FA.
Phytosterols, including beta-sitosterol and its glucoside beta-sitosterolin, closely resemble the molecule cholesterol. These molecules interfere with cholesterol absorption in humans. The lowered absorption of cholesterol from the intestines decreases low density lipoprotein (LDL), which reduces plasma cholesterol levels.
For the purposes of the present invention, phytosterols include, for example but are not limited to, beta-sitosterol, sitostanol, campesterol, campestanol, stigmasterol, stigmastanol, brassicasterol, and other compounds containing the sterol ring system. As used herein, “total sterols” include, for example but not limited to, all of the phytosterols described herein. As used herein, phytosterols also include sterol glucosides, sterol fatty acid esters, and sterol ferulate esters.
The method disclosed in Moreau et al. differs significantly from the present invention in that the extractions described in the present invention are completed on high-solids, thermochemically treated corn fiber. Also, the extractions of the process of the present invention may be carried out on either dry corn fiber or wet, unground corn fiber. The lack of need for completely dry corn fiber is an advantage of the process of the present invention because the energy needed to reduce the fiber from 65% water (typical for mechanically dewatered corn fiber) to 0% unbound water is high. Additionally, grain dust explosions are a potential hazard for grain storage and milling operations. By processing wet, unground corn fiber, the chance of a grain dust explosion is minimized. Concentrations from 0%/100% water/ethanol to 30%/70% water/ethanol of the extractant are used in the process of the present invention to extract the phytosterols. If the water content of the corn fiber after thermochemical treatment is 65%, then 325% of the weight of the total corn fiber stream will need to be added in anhydrous ethanol to achieve an 80%/20% solvent. Therefore, if the amount of water present in the corn fiber can be reduced prior to extraction, the ethanol usage will be greatly decreased.
Ethanol and ethanol/water mixtures are most compatible with current corn wet-milling plants that produce ethanol, since both ethanol and water are available for use in such plants.